As turbines for turbochargers mounted on passenger cars, low-cost radial turbines have been mainly used. As shown in FIG. 11, in a radial turbine 100, a shaft end of a rotary shaft 102 is mounted with a turbine wheel 104. The turbine wheel 104 is constituted by a turbine disk 105 fixed to the rotary shaft 102, and a plurality of rotor blades 106 are provided to protrude outward in a radial direction from the turbine disk 105. A turbine housing 108 is provided so as to cover the turbine wheel 104. The turbine housing 108 is attached to a bearing housing 116 adjacent thereto with a cap 118.
The turbine housing 108 is constituted by a scroll portion 110 that has a spiral shape, and a cylindrical portion 112 that communicates with a spiral scroll chamber s formed inside the scroll portion 110 in a communication portion d. The turbine housing 108 on the upstream side of the rotor blades 106 is provided with an opening o that allows the rotor blades 106 to pass therethrough, and a back plate 114 that shields the opening o is detachably attached to the turbine housing 108. By removing the back plate 114, the rotary shaft 102 with the turbine wheel is enabled to leave and enter the turbine housing 108 through the opening o.
A swirling flow c that is directed to a circumferential direction of the turbine wheel 104 is formed in the scroll chamber s. The scroll portion 110 of the radial turbine 100 has a relatively large external diameter, and the area of the communication portion d is constricted. Accordingly, the swirling flow c within the scroll chamber s enters the inside of the cylindrical portion 112 at a large flow velocity, and rotates the rotor blades 106. Since the swirling flow c is formed within the scroll chamber s in this way, stator blades are unnecessary, and—cost reduction can be realized.
Meanwhile, in the radial turbine 100, in order to efficiently transmit the swirling force of the swirling flow c to the rotor blades 106, the rotor blades 106 are formed with swelling regions a that swell out outward in a radial direction toward the scroll chamber s. Therefore, a large moment of inertia is generated at the swelling regions a where the peripheral velocity increases during the rotation of the rotor blades 16.
FIG. 12 shows an axial-flow turbine. In an axial-flow turbine 120, a turbine wheel 124 is provided at a shaft end of a rotary shaft 122, and the turbine wheel 124 is constituted by a turbine disk 125 and rotor blades 126. A turbine housing 128 is provided so as to cover the turbine wheel 124, and the turbine housing 128 is constituted by an annular fluid inflow portion 130, and a cylindrical portion 132 that communicates with an inflow space S formed within the fluid inflow portion 130 through a communication portion d. Stator blades 136 are provided on an inner wall of the cylindrical portion 132 on the upstream side of the rotor blades 126. The communication portion d of the inflow space S is not constricted, forms a swirling flow c with the stator blades 136, and rotates the rotor blades 126 by the swirling flow c.
The axial-flow turbine is used for large-sized turbochargers for ships or the like, and the costs of it is high because the stator blades 136 has to be provided. However, since the axial-flow turbine forms the swirling flow with the stator blades 136, the scroll portion 130 can be made small. Further, since the rotor blades 126 are arranged inside the cylindrical portion 132, there is no generation of a large moment of inertia in the rotor blades 126.
Acceleration response at the start of vehicle is important for turbochargers mounted on passenger cars or the like, particularly on light vehicles. Therefore, an efficient rotation using a low flow rate of exhaust gas of the turbine wheel is necessary. Also, it is necessary to reduce the overall size and weight the turbochargers including the turbine wheel. In contrast, since the radial turbine has the swelling regions a at the rotor blades and the external diameter thereof becomes large, a large moment of inertia is generated. Therefore, there is a problem in that the response is not good. Meanwhile, since the stator blades are provided in the axial-flow turbine, it is necessary to insert the rotor blades from an outlet direction (a direction of arrow b in FIG. 12) during assembly, and the costs rise. Moreover, there are problems such that the dimension of the cylindrical portion 132 becomes large in the axial direction of the rotary shaft 122.
PTL 1 discloses a radial-type turbine for a turbocharger in which the reduction in size and weight is achieved to be used in passenger cars or the like. This turbine for a turbocharger will be simply described with reference to FIG. 13. In FIG. 13, in a turbine 140 for a turbocharger, a turbine wheel 144 fixed to a shaft end of a rotary shaft 142 is fixed. The turbine wheel 144 is constituted by a turbine disk 145 fixed to the shaft end of the rotary shaft 142, and a plurality of rotor blades 146 are provided to protrude outward in a radial direction from the turbine disk 145. A turbine housing 148 is provided so as to cover the turbine wheel 144, and the turbine housing 148 is constituted by a scroll portion 150 that has a spiral shape, and a cylindrical portion 152 in which the rotor blades 146 are accommodated.
In the turbine 140 for a turbocharger, the swelling regions that swell out outward in the radial direction are eliminated from leading edges 146a of the rotor blades 146, the leading edges 146a of the rotor blades 146 are formed obliquely with respect to the axial direction of the turbine wheel 144, and the inclination angle α thereof is made as large as 35° to 60°. Accordingly, the moment of inertia of the rotor blades 146 can be reduced, the centrifugal force of the rotor blades 146 is reduced, and the inflow of exhaust gas or the like from the scroll chamber S is made easy. Additionally, the rotor blades 146 are arranged inside the cylindrical portion 152, and the external diameter D thereof is made constant, whereby the turbine wheel 144 is enabled to leave and enter from an axial direction (a direction of arrow b) thereof during assembly. This allows the integration of the turbine housing 148. PTL 2 also discloses a turbine for a turbocharger that has the same configuration as that of PTL 1.